The present invention relates to a water dispersible material whose solubility in water is dependent upon the total ionic concentration in the water and particularly the concentration of divalent ions. More particularly, the invention relates to a polymer binder composition that is dispersible in water when the concentration of divalent ions in the water is less than about 50 parts per million (ppm) and desirably, the concentration of monovalent ions is less than about 0.4 weight percent. Advantageously, the polymer composition is insoluble in an aqueous solution having a concentration of divalent ions greater than about 50 ppm. The invention is further directed to a method of making a water dispersible nonwoven fibrous fabric comprising a fibrous substrate and the ion sensitive binder composition distributed therein and the water dispersible nonwoven fibrous fabric utility in water-dispersible personal care products.
While the composition and products of the present invention are described herein primarily in connection with advantageous applications as a disposable absorbent article and more particularly, a premoistened wipe, it should be understood that the present invention is not limited thereto. In light of the present disclosure, those skilled in the art will recognize a variety of applications in other fields where flushability of a fibrous fabric would be desirable.
Nonwoven fibrous fabrics and webs are widely used as components of disposable products as sanitary napkins, diapers, wound dressings, bandages, nursing pads, and premoistened wipes. The terms "nonwoven fibrous webs", "fibrous webs", "nonwoven fabrics", "fabrics" and "fibrous substrates" are interchangeably used herein and include, without limitation, methods of making such fabrics and webs which can include, but are not limited to, air-laying and wet laying.
Such fabrics, if they are to function effectively, must maintain their structural integrity as well as exhibit satisfactory tensile strength when they are damp or wet. However, it has been recognized that if such nonwoven fabrics were to lose substantially all of their tensile strength when exposed to water and become readily dispersible therein, disposal problems would be substantially eliminated. The products could be easily and conveniently flushed down a conventional toilet or water closet.
Desirably, the fabrics possess a number of characteristics such as softness and flexibility. The fabric is usually formed by wet or dry laying random fibers and joining them together to form a coherent web. In an attempt to provide a nonwoven fabric with specific in-use characteristics, prior methods have rendered the fabric non-dispersible in water. For example, nonwovens have been bonded with fluid-insoluble resins which impart in-use strength. However, such resins impede flushing and dispersal of the fabric by rendering the fabric substantially water insoluble.
With regard to premoistened wipes, special problems arise. The wipes, which are used for skin cleansing and are known commercially as towelettes, wet wipes or fem-wipes, are formed from paper or nonwoven fibrous webs treated with a polymeric binder. The binder imparts to the web a degree of wet strength so that the web will not lose its tensile strength while being stored in an appropriate liquid medium. However, after the wipe has been used, the binder should be readily weakened when exposed to an aqueous environment, such as when the wipe is disposed of in a toilet, without clogging the toilet and plumbing.
In an attempt to impart dispersibility in water, various binders have been used in the manufacture of the wipe. For example, wipes have included as a binder an acid-insoluble, alkali-soluble polymeric polycarboxylic acid and functional derivatives thereof wherein the acid is placed in water and enough alkali is added to substantially neutralize all acidic groups prior to applying the binder to the web. The binder-saturated web is dried and then immersed in a low pH medium where it retains its structural integrity yet will still break up when the wipe is immersed in a sufficiently high pH liquid medium.
Another binder used has been polyvinyl alcohol combined with a gelling or insolubilizing agent such as borax. The borax cross-links at least the surface of the polymer binder before the web is dried to give a water resistant web. Such cross-links are reversible, that is, when the concentration of borax is reduced below a certain level, the degree of cross-linking is so low that the binder becomes soluble in water.
A problem with the above binders is that to prevent the nonwoven fibrous fabric from disintegrating prior to disposal, the wipe must be maintained in a solution having a pH which may cause irritation to the skin when the wipe is used.
Another binder that is, to a limited degree, water soluble comprises an unsaturated carboxylic acid/unsaturated carboxylic acid ester copolymer. The web is water soluble, water dispersible or water disintegratable in an aqueous environment, provided the water is predominantly void of divalent ions. However, in those areas where the water is "moderately hard", because the water contains divalent ions such as calcium ions and/or magnesium ions, the wipes do not readily disperse. The water soluble polymeric binder is substantially rendered insoluble by the presence of these divalent ions. Although not wanting to be held to any particular theory, it is believed that the divalent ions irreversibly cross-link the binder, preventing it from dispersing in water. Until now, it has not been recognized the adverse effect that divalent ions present in the aqueous environment has on the water solubility of the polymeric binder.
A problem with the aforementioned binder is that they require a relatively long contact time for the storage solution to wet the fibrous fabric during the high speed converting process. Such times may be as long as several hours. This limits the commercial utility of the binder for use in a moistened wipe.
Another problem with the aforementioned binder is that it does not have sufficient end use strength for use as a wiper.
Accordingly, there is a need for a water soluble binder composition that can be used in an article, such as a wipe, that is safe to use and will be substantially unaffected by the presence of divalent ions normally found in moderately hard water.